content: /**
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
   * compliance with the License. You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software distributed under the License is
   * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
   * the License for the specific language governing permissions and limitations under the License.
   */
  
  package io.reactivex.observable.observers;
  
  import io.reactivex.common.Disposable;
  import io.reactivex.common.annotations.NonNull;
  import io.reactivex.common.internal.disposables.DisposableHelper;
  import io.reactivex.observable.Observer;
  import io.reactivex.observable.internal.utils.EndObserverHelper;
  
  /**
   * Abstract base implementation of an {@link io.reactivex.observable.Observer Observer} with support for cancelling a
   * subscription via {@link #cancel()} (synchronously) and calls {@link #onStart()}
   * when the subscription happens.
   *
   * <p>All pre-implemented final methods are thread-safe.
   *
   * <p>Use the protected {@link #cancel()} to dispose the sequence from within an
   * {@code onNext} implementation.
   *
   * <p>Like all other consumers, {@code DefaultObserver} can be subscribed only once.
   * Any subsequent attempt to subscribe it to a new source will yield an
   * {@link IllegalStateException} with message {@code "It is not allowed to subscribe with a(n) <class name> multiple times."}.
   *
   * <p>Implementation of {@link #onStart()}, {@link #onNext(Object)}, {@link #onError(Throwable)}
   * and {@link #onComplete()} are not allowed to throw any unchecked exceptions.
   * If for some reason this can't be avoided, use {@link io.reactivex.observable.Observable#safeSubscribe(io.reactivex.observable.Observer)}
   * instead of the standard {@code subscribe()} method.
   *
   * <p>Example<pre><code>
   * Observable.range(1, 5)
   *     .subscribeWith(new DefaultObserver&lt;Integer&gt;() {
   *         &#64;Override public void onStart() {
   *             System.out.println("Start!");
   *         }
   *         &#64;Override public void onNext(Integer t) {
   *             if (t == 3) {
   *                 cancel();
   *             }
   *             System.out.println(t);
   *         }
   *         &#64;Override public void onError(Throwable t) {
   *             t.printStackTrace();
   *         }
   *         &#64;Override public void onComplete() {
   *             System.out.println("Done!");
   *         }
   *     });
   * </code></pre>
   *
   * @param <T> the value type
   */
  public abstract class DefaultObserver<T> implements Observer<T> {
      private Disposable s;
      @Override
      public final void onSubscribe(@NonNull Disposable s) {
          if (EndObserverHelper.validate(this.s, s, getClass())) {
              this.s = s;
              onStart();
          }
      }
  
      /**
       * Cancels the upstream's disposable.
       */
      protected final void cancel() {
          Disposable s = this.s;
          this.s = DisposableHelper.DISPOSED;
          s.dispose();
      }
      /**
       * Called once the subscription has been set on this observer; override this
       * to perform initialization.
       */
      protected void onStart() {
      }
  
  }
  